In the past, the practical use of adhesives with electrical conductivity has been limited to non-pressure sensitive adhesives. Generally, the conductive fillers used to impart conductivity are impractical for most adhesive-based applications either because they are prohibitively expensive (such as gold and silver) or they possess oil absorption characteristics that adversely affect the physical properties of the pressure-sensitive adhesive (PSA).
It is known to add conductive blacks (i.e., carbon) to normally non-conductive PSA's, with the electrical properties of the PSA improving with the degree of carbon loading. See, for example, U.S. Pat. No. 3,778,306 which provides for the presence of between about 20-43 percent by weight of carbon in a pressure sensitive adhesive. However, at loadings high enough to provide good conductivity, a reduction in physical performance of the PSA characteristically results, affecting physical properties such as tack, peel, and shear. In the PSA formulation disclosed in the patent, loss of tack due to the presence of large amounts of conductive carbon is compensated for by the addition of a tackifying or plasticizing agent to the formulation and milling the rubber to effect softness.
Loss of adhesive tack is also recognized as a potential problem with respect to electrically-conductive pressure sensitive adhesives in U.S. Pat. No. 2,670,306. In this patent, the use of powdered graphite is stated to be preferred over carbon black to minimize the degree of tack loss.
U.S. Pat. No. 3,475,213 discloses an electrically-conductive adhesive tape which comprises a pressure sensitive adhesive and electrically-conductive particles distributed as a monolayer in the adhesive layer. The particles are stated to have a thickness slightly less than the thickness of the adhesive layer. The patent states that such adhesive tapes exhibit electrical resistances less than 100 ohm/square inch.
Various methods are known to impart conductivity to adhesives. U.S. Pat. No. 2,808,352 discloses an electrically conductive adhesive tape wherein the adhesive base is impregnated with finely divided silver particles. U.S. Pat. No. 4,367,745 discloses a conformable electrically conductive composition composed of a plurality of deformable, non-polar microspheres around which are dispersed electrically conductive particles.
U.S. Pat. No. 4,547,311 discloses an electrically-conductive polymeric coating composition comprising a dispersion of carbon black in an organic solvent and a solution of a polymer in an organic solvent. The coating composition may be used to form coatings of low resistivity. Fluoroelastomers, urethane elastomers and vinylidene fluoride/tetrafluoroethylene polymers are disclosed as exemplary polymers forming the coating composition. Pressure sensitive adhesive compositions are not disclosed.
The '311 patent requires that the dispersion of carbon black in the solvent exhibit a viscosity in the range of 100 to 5000 cp, with the patent examples indicating that significant ball milling of the dispersion is required to achieve this result.
While ball milling pigment in a solvent vehicle is a well-known method of improving particle dispersion, it is believed that such ball milling is detrimental to the inherent chain-forming structure of the carbon black. While the patent examples indicate that an electrically conductive film is successfully produced, the resulting electrical properties are believed to be the inherent result of the presence of carbon black in an amounts of 40 and 60 percent by weight based on the weight of the carbon black and the polymer matrix.
U.S. Pat. No. 4,548,862 is directed to a flexible tape having bridges of electrically conductive particles extending through the adhesive layer. The particles have ferromagnetic cores which by magnetic attraction can serve to form the requisite bridges.
U.S. Pat. No. 4,588,762 discloses a pressure sensitive adhesive composition consisting of both a viscoelastic polymeric adhesive phase and an electrically conductive aqueous phase containing a water receptive polymer, a humectant, and an electrolyte.
However, to date no one has provided a pressure-sensitive adhesive composition which exhibits desirable electrical properties (such as high conductivity) by inclusion of conductive carbon black while minimizing the effect of such filler on the adhesive properties of the composition. The prior art to date has also failed to provide a pressure sensitive adhesive composition exhibiting electrical properties which can be easily tailored to a desired end use ranging from static protection to serving as an electrical conductor between electrode surfaces.
The prior art has also failed to provide a conductive pressure sensitive adhesive wherein the conductive carbon black is present in a form which optimizes its ability to enhance the electrical properties of the adhesive.